The broad long term objective of the proposed research is to gain insight into the biological role of gamma delta T cell receptor (TCR) bearing lymphocytes. In part, lymphocyte functions are defined by the ligands their receptors can recognize. We have therefore undertaken to identify ligand for gamma delta TCRs and to define mechanisms of receptor-ligand interactions. In the proposed studies, we follow up on reports by others that an entire subset of (human) gamma delta T cells responds specifically to small non-peptidic ligands whose common denominator is their content of phosphate groups. The mechanisms and physiological relevance of this response have remained unclear, although the mycobacterial origin of some of these ligands implies a role in host resistance. We have recently found a response of murine gamma delta T cells (hybridonmas and clones) to similar anionic ligands, including certain phospholipids, polyanionic peptides and deoxyribonucleic acids. This murine response is likely analogous to the the human response, so that the murine system can be used as a model to assess physiological relevance through in vivo experimentation. Our studies on mouse gamma delta T cell responses have already revealed several intriguing features. Firstly, like the human response, the murine response involves an entire subset gamma delta T cell subset. A common denominator of the reactive cells is their expression of Vgamma1. Vgamma1contains peculiar clusters of basic amino acids which could provide binding sites for anionic ligands. Mutation of one such cluster reduced responsiveness to a phospholipid while not affecting other TCR-dependent reactivity. We now propose to investigate in more detail the role of the TCR in the response to anionic ligands. Secondly, the murine response does not appear to require ligand presentation, but is strictly dependent on the presence of serum. At least two serum factors must be involved, on that is required for the ligand response and one that is inhibitory. It is thus conceivable that serum factors control normal gamma delta T cell reactivity. We therefore propose to characterize the serum factors involved and to define their role in the gamma delta T cell response. Since most of our experiments so far have involved only hybridomas and clones, we also propose to establish conditions for reactivity of normal gamma delta T cells with anionic ligands. Thirdly, gamma delta T cells reportedly can support antibody switching, and the development of autoreactive antibodies in vivo. Having found that certain murine gamma delta T cells respond to DNA and other anionic ligands, we plan to inquire whether the same cells also promote the development of anti DNA and anti phospholipid antibodies.